UNIVERSITY    OF     CALIFORNIA 

COLLEGE   OF   AGRICULTURE 

AGRICULTURAL    EXPERIMENT   STATION 

BERKELEY,  CALIFORNIA 

CIRCULAR  296 

Eevised  May,  1925 

September,  1925 

CONTROL  OF  THE  CALIFORNIA  GROUND  SQUIRREL 

JOSEPH  DIXON 
(Contribution  from  the  Museum  of  Vertebrate  Zoology  of  the  University  of  California) 


Fig.  1. — Diagram  showing  methods  of  destroying  ground  squirrels  to  be  employed 
at  different  seasons  of  the  year. 


The  ground  squirrels  are  undoubtedly  the  most  widely  known  and 
at  the  same  time  among  the  most  destructive  mammals  in  California. 
The  large,  long-tailed,  grayish-brown  species  known  as  the  California 
or  "digger"  squirrel  (Citellus  beecheyi  and  subspecies)  is  the  most 
important  in  relation  to  man;  first,  because  of  its  destructiveness  to 


2  UNIVERSITY    OP    CALIFORNIA — EXPERIMENT   STATION 

crops;  second,  because  it  is  a  carrier  of  disease;  and  third,  because 
of  the  damage  done  by  it  to  irrigation  works. 

Three  races  of  the  digger  squirrel  occur  within  the  state :  the  dark- 
colored  northern  form  (the  Douglas  ground  squirrel)  which  ranges 
from  San  Francisco  Bay  northward  throughout  that  portion  of  the 
state  which  lies  west  and  north  of  the  Sacramento  and  Feather  rivers ; 
the  brownish  Beechey  ground  squirrel  which  occupies  central  Califor- 
nia and  the  coast  district  south  from  the  Golden  Gate  clear  to  the 
Mexican  line ;  and  the  gray-toned  Fisher  ground  squirrel  whose  habitat 
lies  in  the  southern  San  Joaquin  and  Owens  valleys  and  south  along  the 
western  border  of  the  Mohave  Desert.  The  characters  which  serve  to 
distinguish  these  different  races  are  not  of  any  particular  significance 
in  relation  to  the  work  of  eradication,  so  that  from  this  standpoint 
the  three  forms  may  be  considered  as  one.  A  knowledge  of  certain 
habits  of  feeding,  breeding  and  hibernation,  however,  possessed  in 
common  by  these  animals  is  of  the  greatest  importance  in  control  work 
and  these  habits  are  considered  at  some  length  in  the  following  pages. 

NATURE   AND    EXTENT  OF    DAMAGE 

In  1920,  Mr.  F.  E.  Garlough,  of  the  United  States  Biological 
Survey,  placed  the  average  annual  loss  due  to  ground  squirrels  in 
California  at  twelve  million  dollars.  Allowing  for  the  decline  in 
prices  since  1920,  it  is  the  author's  opinion  that  the  annual  loss  due 
to  ground  squirrels  in  this  state  is  now  not  less  than  five  million 
dollars. 

The  California  ground  squirrel  occurs  in  troublesome  numbers  in 
most  of  the  best  farming  and  fruit-raising  districts  of  the  state.  It 
is  a  voracious  feeder  and  reproduces  rapidly.  Many  kinds  of  fruit 
and  practically  all  kinds  of  grain  are  subject  to  its  depredations.  This 
squirrel  prefers  an  elevated  location  for  its  homesite,  and  in  digging 
burrows  it  often  honeycombs  the  banks  of  irrigation  ditches,  with 
disastrous  results. 

The  ground  squirrel  is  a  carrier  and  disseminator  of  bubonic 
plague.  Between  May,  1907,  and  September,  1912,  over  one  thousand 
plague-infected  ground  squirrels  were  examined  from  a  single  county 
in  the  San  Francisco  Bay  region  by  the  United  States  Public  Health 
Service.  That  the  immediate  danger  of  acquiring  the  plague  through 
ground  squirrels  has  not  passed,  is  shown  by  the  virulent  outbreaks 
of  the  disease  in  Alameda  County  in  1918  and  in  Los  Angeles  County 
in  1924.  Their  destruction  in  the  infected  districts  would  tend  to 
diminish  the  frequency  of  recurrence  of  this  danger. 


ClRC.  296]  CONTROL  OF  THE  CALIFORNIA  GROUND  SQUIRREL 


FEEDING    HABITS 

California  ground  squirrels  have  membranous  cheek-pouches  which 
open  inside  the  mouth  and  which  are  used  to  carry  food,  principally 
grain,  to  the  underground  storerooms.  These  cheek-pouches  are  not 
lined  with  fur  as  are  those  of  the  pocket  gopher,  but  have  thin  walls 
which  readily  absorb  the  strychnine  on  the  outside  of  poisoned  grain. 
It  has  been  demonstrated  that  ground  squirrels  are  more  readily 
poisoned  through  these  cheek-pouches  by  merely  carrying  poisoned 
grain  than  through  the  stomach  after  they  have  eaten  it.  For  this 
reason  the  coated  grain  is  preferred  to  that  which  has  been  soaked 
in  a  strychnine  solution. 

The  food  and  feeding  habits  of  ground  squirrels  vary  greatly  with 
the  locality  and  season  of  the  year.1  During  the  rainy  season,  from 
November  to  April,  although  they  eat  some  grain,  seeds  or  nuts  that 
have  been  stored  during  the  previous  summer,  their  main  food  is  grass 
and  other  green  herbage.  This  results  in  serious  loss  in  many  locali- 
ties, by  reducing  greatly  the  pasturage  available  for  horses  and  cattle. 
By  weighing  the  food  eaten  by  squirrels,  I  have  found  that  200  ground 
squirrels  on  the  open  range  consume  as  much  green  forage  as  one 
steer,  and  20  as  much  as  one  sheep.  When  green  stuff  is  abundant, 
comparatively  little  food  is  carried  in  the  cheek-pouches.  However, 
by  the  last  of  March  certain  plants,  such  as  alfilaria,  have  begun  to 
go  to  seed,  and  the  squirrels  then  begin  to  use  their  cheek-pouches 
extensively  in  harvesting  these  seeds.  At  this  time  the  squirrels  take 
the  poisoned  grain  readily,  but  later  in  the  season  when  the  cultivated 
grain  begins  to  ripen  they  are  much  harder  to  poison  on  account  of 
the  abundant  grain  supply  then  available. 

Ground  squirrels  are  very  fond  of  the  soft  pits  of  peaches,  almonds 
and  apricots,  and  in  getting  these  they  destroy  much  fruit.  On  the 
whole,  however,  the  principal  loss  from  ground  squirrels  is  in  the 
grain  fields,  where  the  animals  dig  up  and  destroy  the  sprouting  seed 
and  later  pull  down  and  destroy  a  vast  amount  of  the  ripening  grain. 
After  the  crops  are  harvested  they  congregate  about  the  shocks  and 
stacks  and  continue  to  devour  and  carry  off  quantities  of  grain.  At 
this  time  watermelon  rinds  poisoned  with  strychnine  are  extremely 
effective,  since  succulent  green  food  is  scarce. 


i  For  a  detailed  account  of  general  habits,  see:  Grinnell,  J.,  and  Dixon,  J. 
Natural  history  of  the  ground  squirrels  of  California.  Mo.  Bull.  State  Comm. 
Hort.  (Sacramento),  7,  nos.  11-12,  597-708,  plates  5,  1918. 


UNIVERSITY    OF    CALIFORNIA — EXPERIMENT    STATION 


AESTIVATION   OF  THE   GROUND   SQUIRREL 

There  is  evidence  that  a  period  of  aestivation,  or  torpidity  induced 
by  the  dryness  and  accompanying  food  shortage  in  summer,  obtains 
among  some  of  the  adult  ground  squirrels  in  the  valleys  of  California. 
This  period  may  extend  continuously  from  late  summer  well  through 
midwinter,  and  thus  involve  hibernation  as  well  as  aestivation.  Only 
the  old  adults  seem  to  "hole  up,"  for  the  young  adults  (somewhat 
over  a  year  old)  and  the  young  of  the  year  may  be  seen  about  the 
burrows  during  suitable  weather  throughout  the  winter.  A  female 
ground  squirrel  that  lived  under  normal  conditions  in  a  dooryard  was 
kept  under  regular  and  continual  observation.  This  squirrel  did  not 
aestivate  until  its  second  year.  Then  and  during  each  succeeding  year 
of  its  life  it  aestivated  regularly,  becoming  very  fat  and  retiring  to  its 
burrow  during  the  last  week  in  August.  When  removed  from  the 
burrow  during  this  period,  it  was  found  to  be  in  a  torpid  state  and 
respiration  was  not  perceptible.  It  emerged  in  an  emaciated  condition, 
with  marked  regularity,  about  the  22nd  of  each  following  February. 
This  habit  of  aestivation  affords  explanation  of  a  case  in  my  experience 
in  which  all  the  squirrels  that  were  active  in  a  certain  field  in  the  fall 
were  poisoned  or  killed,  and  yet  old  breeding  squirrels  suddenly 
appeared  in  this  field  the  following  February.  This  occurred  when 
there  was  seemingly  no  possible  chance  for  re-infestation  from  the 
surrounding  fields.  The  extent  of  this  habit  of  aestivation  among  our 
ground  squirrels  is  unknown.  It  is  exceedingly  difficult  to  follow  any 
individual  squirrel  through  all  its  various  activities  for  any  great 
length  of  time.  However,  it  suggests  the  advisability  of  poisoning  in 
the  spring  rather  than  in  the  fall  when  some  of  the  breeding  stock 
may  be  stowed  away,  out  of  the  reach  of  poisoned  grain.  It  is  a  ques- 
tion, too,  whether  or  not  a  dormant  animal,  in  which  respiration  is 
extremely  slow,  would  be  fatally  injured  by  fumigation  before  the 
gas  disappeared. 

TIME   AND    RATE   OF    BREEDING 

The  accompanying  chart  (fig.  2),  based  on  over  10,000  female 
squirrels  examined  by  the  United  States  Public  Health  Service  during 
the  spring  and  summer  of  1910,  shows  that  the  California  ground 
squirrel  has  a  very  definite  breeding  season  which  begins  about  the 
first  of  February  and  lasts  until  the  first  of  May.  The  maximum 
number  of  pregnant  females  was  found  during  the  week  ending 
February  26.    Most  of  the  young  are  born  about  five  weeks  after  this 


CIRC.  296]  CONTROL  OF  THE  CALIFORNIA  GROUND  SQUIRREL  5 

date  and  may  be  seen  running  about  at  the  entrances  of  the  burrows 
when  a  month  or  six  weeks  old.  In  low,  warm  valleys  the  breeding 
season  has  been  found  to  be  somewhat  earlier,  while  along  the  coast 
and  in  the  mountains  the  breeding  season  is  about  two  weeks  later 
than  the  dates  given. 

The  usual  annual  increase  of  a  pair  of  ground  squirrels  consists  of 
one  litter  of  from  five  to  eleven.  The  average  number  of  embryos 
in  pregnant  females  was  found  by  the  United  States  Public  Health 
Service  to  be  7.2.    W.  C.  Jacobsen2  found  that  on  areas  where  control 


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Fig.  2. — Chart  showing  breeding  season  of  California  ground  squirrels.  (Com- 
piled chiefly  from  data  obtained  from  United  States  Public  Health  Reports,  vol. 
27,  July  5,  1917,  p.  1070.)      Figures  in  left-hand  column  represent  per  cent  of 

females    found    pregnant;    over    10,000    females    examined.      ■ represents 

prevalence  of  pregnancy  among  female  ground  squirrels  between  January   15 
and  May  14.     For  example,  during  the  week  ending  March  5,  27  per  cent  of 

females  examined  were  pregnant.     shows  approximately  the  time  of  birth 

of  the  bulk  of  young  squirrels. 


operations  had  been  carried  on  for  two  years  or  longer,  and  where 
the  food  supply  had  increased  by  the  diminution  of  the  number  of 
squirrels,  the  litters  were  uniformly  larger  than  where  the  work  of 
reducing  the  squirrel  population  had  just  begun.  Thus  in  Tulare 
County,  where  ground  squirrels  had  been  greatly  reduced  in  numbers 
for  a  period  of  years,  Jacobsen  found  that  86  pregnant  females  con- 
tained 846  embryos,  an  average  of  nearly  10  per  litter.     There  is 


2  Jacobson,  W.   C.     Rate   of  reproduction  in   Citellus   oeecheyi.     Journal   of 
Mamm.,  4,  58,  1923. 


6  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT   STATION 

some  evidence  that  a  second  litter  ia  sometimes  raised  in  a  single 
season,  but  this  is  certainly  not  usual.  The  important  point  is  that 
one  breeding  female  killed  before  April  1,  that  is,  before  the  young 
are  able  to  take  care  of  themselves,  is  equal  to  the  destruction  of 
from  six  to  twelve  squirrels  later  in  the  season.  A  wise  man  will  go 
after  the  squirrels  early  in  the  year,  when  "a  stitch  in  time"  will 
literally  "save  nine." 


METHODS    OF    CONTROL 

The  five  most  effective  methods  of  destroying  ground  squirrels  are : 
(1)  poisoning  with  strychnine;  (2)  fumigation  with  carbon  bisulfide 
or  cyanide;  (3)  trapping;  (4)  shooting;  (5)  encouragement  of  the 
natural  enemies  of  the  ground  squirrel. 


WHICH  METHOD  TO  USE,  WHEN,  AND  WHY 
(See  Fig.  1) 

1.  Strychnine-coated  barley  is  best  used  during  the  dry  season 
because  at  this  time  the  squirrels  gather  and  store  grain  and  hence 
are  easily  poisoned  through  their  cheek-pouches  when  in  the  act  of 
carrying  the  poisoned  grain.  Rain  and  heavy  fogs  tend  to  wash  the 
strychnine  off  the  poisoned  grain.  It  has  been  found  by  Mr.  Garlough 
that  where  ground  squirrels  are  eating  green  alfilaria  in  quantity, 
poison  grain  though  eaten  has  little  or  no  effect.  This  is  explained 
by  the  fact  that  this  plant  contains  enough  tannin,  which  is  an 
antidote  for  strychnine,  to  neutralize  the  poison.  In  certain  localities 
acorns  are  said  to  have  a  similar  effect. 

2.  Powdered  strychnine  (sulfate)  in  fresh  vegetables  and  fruit  is 
especially  effective  in  the  dry  season  when  green  food  is  scarce. 

3.  Carbon  bisulfide  or  cyanide  is  most  effective  when  the  soil  is 
damp.  When  the  ground  is  dry  the  gas  escapes  through  the  cracks 
in  the  soil. 

4.  Trapping  and  shooting  are  effective  at  any  time,  but  are  from 
six  to  twelve  times  more  so  before  the  young  are  out,  before  April  1, 
than  later  in  the  season. 

5.  Reel-tailed  hawks,  golden  eagles,  badgers,  weasels,  and  other 
natural  enemies  of  the  ground  squirrel  will  prove  valuable  allies  in 
the  war  on  ground  squirrels  if  they  are  merely  allowed  to  live.  It 
costs  little  to  let  them  alone,  to  go  about  their  business  in  the  natural 
wav. 


ClRC.  296]  CONTROL  OF  THE  CALIFORNIA  GROUND  SQUIRREL 


COMPARATIVE    COST   OF   VARIOUS    METHODS 

In  1910,  Dr.  C.  Hart  Merriam3  gave  the  cost  of  one  treatment  with 
poisoned  grain  as  3  cents  per  acre  and  for  one  treatment  with  carbon 
bisulfide  ("waste-ball""  method)  at  1%  cents  per  burrow. 

In  1912,  Surgeon  John  D.  Long4  puts  the  cost  of  the  various 
methods  of  destruction,  with  carbon  bisulfide  at  90  cents  per  gallon, 
as  follows ;  cost  per  acre,  estimated  on  the  basis  of  ten  holes  per  acre. 

Carbon  bisulfide,  with  "destructor"  (two  treatments) $  .20 

Poisoned  grain  (four  or  rive  treatments) 35 

Carbon  bisulfide,  waste-ball  method  (two  treatments) 68 

Surgeon  Long  reports  that  in  actual  practice  one  gallon  of 
bisulfide  would  treat  from  200  to  250  holes  when  used  with  the 
"destructor"  and  from  50  to  60  holes  when  used  with  the  waste-ball 
method. 

Crude  carbon  bisulfide  is  quoted  (April,  1925)  at  $1.15  per  gallon, 
in  five-gallon  lots  in  San  Francisco.  Strychnine  is  quoted  from  78 
cents  to  93  cents  per  ounce  for  the  sulfate,  and  from  $1.00  to  $1.19 
per  ounce  for  the  alkaloid  form,  according  to  the  quantity  purchased. 
The  price  of  ammunition  is  also  high,  so  that  the  present  cost  of  the 
various  methods  of  destroying  ground  squirrels  is,  on  the  average, 
considerably  higher  than  Surgeon  Long's  estimate,  which  latter  seems 
to  be  the  best  and  most  accurate  thus  far  published  on  the  subject, 
both  as  to  the  number  of  treatments  required  and  as  to  the  relative 
cost. 

(1)   POISONING  WITH  STEYCHNINE 

Barley  as  a  vehicle  for  the  poison  is  usually  more  attractive  to  the 
ground  squirrel  than  wheat.  It  is  also  less  likely  to  be  eaten  by  birds, 
stock  and  poultry.  It  should  therefore  be  used  for  this  purpose 
instead  of  wheat.  Care  should  always  be  taken  not  to  leave  more 
than  a  teaspoonful  of  poisoned  grain  in  a  place  and  it  should  never 
be  left  where  poultry  can  pick  it  up.  The  alkaloid  form  of  strychnine 
is  only  slightly  soluble  in  water  and  is  considered  the  best  form  to 
use  in  preparing  poisoned  barley  for  the  reason  that  it  is  not  so 
easily  affected  by  fog  and  rain.  When  gathered  and  placed  in  the 
cheek-pouches  by  the  ground  squirrel,  the  strychnine-coated  barley 


s  Merriam,  C.  H.     U.  S.  Dept.  Agric,  Biological  Survey,  Cir.  76:1910. 
4  Long,  J.  D.    United  States  Public  Health  Eeports,  27,  no.  39,  September  27, 
1912. 


8  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

often  gets  in  its  deadly  work  before  the  animal  has  a  chance  to  detect 
the  bitter  taste  of  the  strychnine  and  reject  the  poisoned  grain. 

Formula  for  Strychnine-coated  Barley. — The  following  is  the 
regular  government  formula5  for  preparing  poisoned  barley  for 
California  ground  squirrels. 

Barley  (clean  grain) 16  quarts 

Strychnine    (powdered   alkaloid) 1  ounce 

Bicarbonate  of  soda  (baking  soda) 1  ounce 

Thin  starch  paste %  pint 

Heavy  corn  sirup ^4   pint 

Glycerin 1  tablespoonful 

Saccharin y10  ounce 

Mix  thoroughly  1  ounce  of  powdered  strychnine  (alkaloid)  and  1  ounce  of 
common  baking  soda.  Sift  this  into  %  pint  of  thin,  hot  starch  paste  and  stir  to 
a  smooth,  creamy  mass.  (The  starch  paste  is  made  by  dissolving  1  heaping  table- 
spoonful  of  dry  gloss  starch  in  a  little  cold  water,  which  is  then  added  to  %  pint 
of  boiling  water;  boil  and  stir  constantly  until  a  clear,  thin  paste  is  formed.) 
Add  y±  pint  of  heavy  corn  sirup  and  1  tablespoonful  of  glycerin  and  stir  thor- 
oughly. Add  y10  ounce  of  saccharin  and  stir  thoroughly.  Pour  this  mixture  over 
16  quarts  of  clean  barley  and  mix  well  so  that  each  grain  is  coated. 

Caution. — All  containers  of  poison  and  all  utensils  used  in  the  preparation  of 
poisons  should  be  kept  PLAINLY  LABELED  and  OUT  OF  BEACH  of  children, 
irresponsible  persons,  and  livestock. 

Placing  Poisoned  Grain.— The  poisoned  barley  should  be  well 
scattered  over  clean  hard  ground  near  the  holes.  Squirrel  paths  that 
lead  along  fences  or  to  or  from  the  dens  are  good  places  in  which  to 
scatter  the  grain.  Poisoned  grain  will  be  largely  wasted  if  dropped  in 
thick  grass  or  in  dusty  places.  It  will  also  often  be  covered  up  if  left 
on  the  loose  dirt  at  the  entrance  to  the  burrow. 

Poisoning  Fresh  Fruits  and  Grain. — Strychnine  sulfate  is  freely 
soluble  in  fruit  juices  and  these  seem  to  conceal,  to  some  degree  at 
least,  the  bitterness  of  the  strychnine.  Some  of  the  best  results  in  the 
writer's  experience  in  poisoning  squirrels  were  secured  with  oranges 
cut  in  halves  which  were  then  sprinkled  with  powdered  strychnine  and 
left  in  the  paths  of  the  squirrels.  Watermelon  rinds,  if  cut  into  pieces 
as  large  as  one's  hand  and  poisoned  by  sprinkling  with  powdered 
strychnine,  often  prove  very  effective.  It  is  wise  to  take  every  pre- 
caution to  place  these  baits  in  protected  places,  such  as  rockpiles, 
where  there  is  no  chance  for  stock  to  reach  them ;  and  it  is  also 
advisable  thoroughly  to  wash  one's  hands  immediately  after  putting 
out  the  poisoned  bait. 

5  Circular  letter  issued  by  Bureau  of  Biological  Survey,  U.  S.  Dept.  Agric, 
June,  1917. 


ClRC.  296]  CONTROL  OF  THE  CALIFORNIA  GROUND  SQUIRREL 


(2)   FUMIGATION   WITH    CARBON    BISULFIDE    OR    CYANIDE 

Carbon  bisulfide  gas  is  one  of  the  best  agents  for  destroying  those 
ground  squirrels  that  have  failed  to  take  the  poisoned  grain  or,  having 
once  survived  the  poison,  refuse  to  take  it  again.  The  following 
facts  should  be  kept  in  mind  regarding  the  use  of  carbon  bisulfide. 
(a)  The  gas  from  carbon  bisulfide  is  highly  inflammable  and  it  should 
be  kept  away  from  all  fires  and  exposed  lights,  (b)  Being  heavier 
than  air,  this  gas  settles  in  the  lowest  places  in  the  underground 
burrow  and  hence  will  not  go  over  an  elevation  higher  than  the 
entrance  of  the  burrow  unless  an  "exterminator"  or  ' ' destructor ' ' 
is  used  forcibly  to  pump  the  burrow  full  of  gas  or  unless  the  gas  is 
exploded  in  the  burrow,  (c)  When  the  ground  is  dry  and  full  of 
cracks  the  gas  escapes  and  is,  therefore,  not  nearly  so  effective  as  if 
used  when  the  ground  is  wet  and  the  gas  thus  confined  to  the  burrow. 
(d)  It  should  be  used  only  in  holes  that  are  known  to  be  occupied  by 
squirrels  at  the  time  of  the  treatment,  (e)  Carbon  bisulphide  should  be 
kept  tightly  corked  as  it  loses  strength  rapidly  on  exposure  to  the  air. 

The  two  best  methods  of  applying  carbon  bisulfide  are  by  the  use 
of  the  "waste-ball"  method  and  of  the  "destructor."  The  common 
waste-ball  method  is  to  pour  a  tablespoonful  of  carbon  bisulfide  on  a 
piece  of  cotton  waste,  corncob,  horse  manure,  or  other  absorptive 
material,  which  should  then  be  thrown  as  far  down  the  hole  as  possible 
and  the  opening  immediately  closed  with  earth. 

Exploding  the  Gas. — The  explosion  of  the  gas  in  connection  with 
the  waste-ball  method  is  recommended  where  the  ground  is  damp  and 
there  is  no  danger  from  fire.  A  "destructor"  should  be  used  in  grain 
fields  or  in  other  places  where  there  is  danger  from  fire,  since  by  its 
use  the  burrow  is  pumped  completely  full  of  the  carbon  bisulfide  gas 
and  the  explosion  of  the  gas  is  unnecessary. 

A  six-foot  piece  of  %-inch  pipe  with  one  end  closed  and  pointed 
and  the  other  end  tightly  wrapped  with  a  rag  soaked  in  coal  oil  adds 
safety  to  the  operation  of  exploding  the  gas  in  the  burrow.  After 
the  bisulfide  has  been  confined  in  the  burrow  for  a  few  seconds  a  hole 
is  made  with  the  pointed  end  of  the  pipe  through  the  dirt  plug  at  the 
entrance  of  the  burrow.  The  gas  is  then  "touched  off"  with  the 
torch,  which  has  been  previously  lighted,  on  the  other  end  of  the  rod. 
It  is  advisable  to  stand  well  to  one  side  of  the  burrow  when  doing  this. 

The  second  system  of  applying  carbon  bisulfide  has  been  used  by 
Passed  Assistant  Surgeon  John  D.  Long  of  the  U.  S.  Public  Health 
Service,    who    devised    a    simple    "destructor"    which    pumps    the 


10  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

vaporized  bisulfide  into  the  burrow.6  Laboratory  experiments  showed 
that  the  animal  is  but  little  alarmed  by  the  gas  and  makes  no  effort 
to  escape  or  to  seek  fresh  air.  In  ten  or  fifteen  minutes  after  the  gas 
is  pumped  into  the  burrow  the  animal  falls  over  and  in  from  thirty 
to  forty-five  minutes  is  dead. 

This  destructor  (fig.  3)  is  composed  principally  of  18-gauge  galvanized  iron. 
The  circular  top,  bottom  and  piston  are  made  from  veneered  wood  so  as  to  pre- 
vent warping  or  splitting.  The  main  parts  of  this  apparatus  are  as  follows: 
An  air  chamber  (1),  containing  the  piston  (10),  is  entirely  surrounded  by  the 
bisulfide  tank  (2).  This  tank  is  connected  with  the  vaporizing  chamber  (4) 
through  the  measuring  cup  (15).  In  operating  the  destructor,  a  short  rubber 
hose  connected  to  the  outlet  (17)  is  inserted  at  least  one  foot  into  an  occupied 
squirrel  burrow  and  tightly  surrounded  with  dirt.  Then  the  three-way  cock  (16) 
is  turned  so  that  the  measuring  cup  (15)  is  filled  with  y2  ounce  of  refined 
bisulfide,  which  is  permitted  by  another  turn  of  the  cock  to  run  into  the 
vaporizing  chamber   (4). 

The  pump  has  a  double  action,  for  air  is  admitted  at  the  proper  time  at  the 
inlet  valves  (7)  at  both  the  top  and  the  bottom  of  the  cylinder  when  the  piston 
(10)  is  worked  up  and  down  by  the  handle  (14).  The  air  thus  compressed  escapes 
through  the  outlet  valves  (8)  into  the  air  shaft  (9),  which  in  turn  conveys  it 
to  the  vaporizing  chamber  (4).  From  the  vaporizing  chamber  the  bisulfide  gas 
is  forced  into  the  burrow  through  a  rubber  hose  connecting  with  the  outlet  (17). 
Fifteen  double  strokes  of  the  pump  forces  12  cubic  feet  of  1.5  per  cent  bisulfide 
gas  down  the  squirrel  burrow;  this  is  sufficient  to  kill  the  animal.  The  rubber 
hose  is  then  withdrawn  from  the  burrow  and  the  hole  closed  by  stamping  in 
the  dirt. 

This  destructor  has  been  placed  on  the  market  through  a  San 
Francisco  firm.  The  Eureka  Squirrel  Exterminator  and  other  similar 
machines  operate  on  principles  similar  to  that  of  the  destructor  just 
described,  while  a. rancher  of  mechanical  turn  of  mind  should  be  able 
to  construct  a  machine  for  the  most  part  out  of  an  old  brass  cylinder 
and  other  materials  from  the  scrap  pile  to  be  found  about  the  average 
ranch.  The  present  high  cost  of  farm  labor  favors  the  use  of  the 
''waste-ball"  rather  than  the  ''destructor." 

Cyanide  in  the  form  of  flakes,  coarse  powder,  or  dust  which 
liberates  the  poisonous  hydrocyanic  acid  gas  has  recently  been  placed 
on  the  market  for  use  in  gassing  ground  squirrels.  From  1  to  2  ounces 
per  burrow  is  used.  The  material  should  be  placed  down  in  the  hole 
as  far  as  possible,  with  a  long-handled  spoon.  The  burrow  should  then 
be  stuffed  with  grass  or  a  newspaper  and  covered  with  earth.  Care 
must  be  exercised  by  the  operator  not  to  breathe  the  poison  gas.  This 
material  can  be  had  in  65-pound  cans  at  from  18  to  23  cents  a  pound. 


e  Long,  J.  D.    United  States  Public  Health  Reports,  27,  no.  39,  September  27, 
1912. 


ClRC.  296]  CONTROL  OF  THE  CALIFORNIA  GROUND  SQUIRREL 


11 


Fig.  3. — Sectional  view  of  squirrel  "Destructor."  (Drawing  from  blueprint 
furnished  by  United  State  Public  Health  Service.)  1,  air  cylinder;  2,  bisulfide 
tank;  3,  filler;  4,  vaporizing  chamber;  5,  veneer  wood  circles;  6,  air  space  to  lower 
valves ;  7,  inlet  air  valves ;  8,  outlet  air  valves ;  9,  air  shaft ;  10,  piston  head ; 
11,  piston  rod;  12,  lock  nuts;  13,  cup  leather  packing;  14,  handle;  15,  measuring 
cup;  16,  three-way  cock;  17,  hose  outlet;  18,  air  inlet  to  vaporizing  chamber; 
19,  foot  rest;  20,  air  vent  in  measuring  cup;  21,  air  deflector;  22,  bottom  board. 


12  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Some  ranchers  report  that  they  find  it  easier  to  use  cyanide  flakes 
than  carbon  bisulfide  and  that  the  percentage  of  squirrels  killed  is 
nearly  or  quite  as  good.  In  orchards  cyanide  is  preferable  to  carbon 
bisulfide.  However,  in  a  series  of  tests  reported  early  in  1925  by 
Mr.  F.  E.  Garlough,  of  the  United  States  Biological  Survey,  carbon 
bisulfide  was  found  still  to  be  the  cheapest  and  best  fumigant  for 
general  use  in  killing  ground  squirrels. 

(3)    TRAPPING;    (4)   SHOOTING 

Where  ground  squirrels  are  digging  into  ditch  banks,  and  in 
similar  cases  where  they  must  be  disposed  of  promptly  at  any  cost, 
special  means  of  getting  rid  of  them  must  be  adopted.  Trapping 
and  shooting  are  two  valuable  methods  of  control  for  such  local  appli- 
cation. While  these  two  methods  can  be  used  at  any  season,  the  time 
required  to  keep  the  traps  properly  set,  which  is  essential  to  success, 
and  the  high  initial  cost  of  the  traps  as  well  as  the  present  high  cost 
of  ammunition  make  them  too  expensive  for  general  use  on  large 
acreage.  Trapping  and  shooting  are  useful  in  cleaning  up  the  few 
wise  squirrels  which  escape  the  poison  and  carbon  bisulfide,  and  in 
reducing  the  breeding  stock  in  the  early  spring  before  the  annual 
five-fold  increase. 

The  No.  91  Oneida  jump  trap  is  the  best,  as  it  is  lighter,  easier  to 
set,  and  has  a  larger  catching  surface  (pan  or  treadle)  than  the 
ordinary  steel  trap  with  the  outside  spring.  The  jump  trap  lies  flat 
on  the  ground;  the  jaws  have  wide  contact  surfaces  which  reduce  the 
chance  of  breaking  the  animal's  leg;  and  the  construction  of  the 
inside  spring  causes  the  trap  to  jump  up  and  take  a  high  grip  on  the 
leg  instead  of  on  the  foot  only.  Such  traps  may  be  set  without  bait 
in  the  entrances  to  the  burrows,  or,  baited  with  rolled  barley,  set  in 
the  squirrel  paths,  or  near  the  places  where  the  squirrels  are  feeding. 
In  any  case  it  is  well  to  scrape  out  by  a  shove  of  the  foot  a  slight 
depression  in  which  to  place  the  trap,  so  that  when  set,  the  upper 
surface  will  be  flush  with  the  surface  of  the  surrounding  ground. 
The  traps  should  be  well  secured.  This  is  conveniently  done  by 
passing  a  three-foot  lath  sharpened  at  one  end  through  the  ring  in 
the  end  of  the  chain,  and  then  driving  the  lath  well  into  the  ground. 
It  will  serve  also  as  a  marker,  enabling  one  readily  to  find  the  trap. 


CIRC.  296]  CONTROL  OF  THE  CALIFORNIA  GROUND  SQUIRREL 


13 


(5)   ENCOUKAGEMENT  OF  THE  NATURAL  ENEMIES  OF  THE 
GROUND  SQUIRREL 

The  killing-off  of  the  natural  enemies  of  the  ground  squirrel 
removes  Nature's  most  effective  check  on  these  destructive  rodents. 
Some  of  these  age-long  natural  enemies  of  the  squirrel  are :  coyotes, 
badgers,  weasels  and  wildcats,  among  mammals;  rattlesnakes  and 
gopher  snakes,  among  reptiles;  red-tailed  hawks  and  golden  eagles, 
among  birds.  Badgers,  weasels  and  snakes  capture  the  ground 
squirrels  in  their  burrows.  Wildcats  and  coyotes  lie  in  wait  near  the 
burrows  until  the  squirrels  venture  forth  in  search  of  food,  when  they 
pounce  upon  them.  Out  of  186  stomachs  of  wildcats  which  have 
recently  been  examined  from  forty  different  localities  in  California, 
26  contained  ground  squirrels,  21  held  wood  rats,  13  contained  gopher 
remains,  and  7  held  mice.  In  this  study  rodents  were  found  to  con- 
stitute more  than  half  of  the  food  of  wildcats.  Hawks  and  eagles 
swoop  down  on  the  squirrels  from  their  vantage  points  in  the  air. 
The  value  of  preserving  as  many  as  possible  of  these  native  enemies 
of  the  ground  squirrel  is  evident  if  this  prolific  animal  is  to  be  kept 
within  bounds.  The  aid  of  coyotes  and  rattlesnakes  may  be  dearly 
bought ;  but  badgers,  gopher  snakes,  hawks  and  eagles  cost  us  little  or 
nothing,  and  each  one  of  these  animals  is  catching  ground  squirrels 
almost  throughout  its  lifetime.  The  following  observations  by  James 
B.  Dixon  show  what  certain  hawks  and  eagles  actually  accomplish  in 
destroying  ground  squirrels.  The  dead  squirrels  counted  in  the  nests 
cited  represent  merely  the  surplus  which  the  old  birds  had  carried  to 
the  young.  The  squirrels  that  the  old  birds  or  the  young  may  have 
eaten  on  the  day  of  observation  are  not  taken  into  account.  Hawks 
and  eagles  it  thus  appears  capture,  when  prey  is  abundant,  far  more 
than  they  and  their  young  can  actually  consume. 


Destructive  Rodents  Found  in  Nests  of  Hawks  and  Golden  Eagle 
in  San  Diego  County 


Species  of  bird 

Date 

Young  and  eggs 

Locality 

Evidence 

Red-bellied 
Hawk 

Apr.     3,  1916 

3  young,    1   week 
old,   and  1  rot- 
ten egg 

Pala 

1  ground  squirrel 
and  2  gophers 

Western  Red- 
tailed  Hawk 

Mar.  28,  1906 

1  day-old  chick,  2 
pipped  eggs  and 
1  rotten  egg 

Vista 

Remains  of  2 
ground  squirrels 

Golden  Eagle 

Mar.  26,  1909 

1  day-old  chick 

Rincon 

9  jack  rabbits 

Golden  Eagle 

Apr.     4,  1907 

2  young,    1   week 
old 

Lilac 

11  ground  squirrels 
in  and  about  nest 

14  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT   STATION 


OTHER  METHODS  OF  CONTROL 

Many  small  deciduous  orchards  are  to  be  found  more  or  less 
isolated  in  foothill  regions  in  different  parts  of  the  state.  Squirrel 
depredations  in  such  orchards  can  often  be  prevented  by  tacking 
smooth  pieces  of  tin  about  the  tree  trunks.  If  the  tinning  is  started 
about  two  feet  above  the  ground  and  continued  upwards  for  two  or 
three  feet  it  will  usually  keep  the  ground  squirrels  out  of  the  trees 
unless  there  are  drooping  branches  which  they  can  climb. 

Seed  corn  has  been  protected  from  ground  squirrels  during 
germination  by  being  treated  with  coal  tar.7  Add  one  large  spoonful 
of  coal  tar  to  a  gallon  of  boiling  water.  When  the  mixture  has  cooled 
somewhat  the  corn  may  be  stirred  in  and  allowed  to  remain  several 
minutes  without  danger  to  germination. 


SOME    DIFFICULTIES    IN    GROUND    SQUIRREL   CONTROL 

One  of  the  most  discouraging  features  in  the  work  of  ground 
squirrel  control  is  the  indifferent,  "do-nothing,"  attitude  of  a  few 
people  in  each  locality  who  make  just  as  little  effort  as  the  law  allows 
to  rid  their  land  of  these  pests.  Many  conscientious  and  progressive 
ranchers  have  justly  asked  why  they  should  continue  to  rid  their 
fields  of  ground  squirrels  when  previous  experience  has  shown  that, 
when  cleaned  up,  these  fields  have  been  promptly  restocked  from  the 
squirrel-infested  lands  of  their  negligent  neighbors.  Now  that  we 
have  an  adequate  squirrel-eradication  law,  there  should  be  less  cause 
for  complaint  on  this  score. 

There  is  no  question  but  that  the  most  effective  campaign  against 
ground  squirrels  is  one  in  which  the  whole  community  joins.  Cooper- 
ation in  this  work  is  the  only  way  in  which  all  the  ground  can  be 
covered,  and  cooperation  is  therefore  essential  to  success.  The  cost  of 
the  war  on  ground  squirrels  to  a  community  can  be  materially  reduced 
by  purchasing  strychnine  and  carbon  bisulfide  in  wholesale  quantities 
and  mixing  its  own  poisoned  grain  under  the  supervision  of  the 
Rodent  Control  Division  of  the  State  Department  of  Agriculture. 
This  will  insure  a  supply  of  uniform,  effective,  and  reliable  poisoned 
grain  for  each  user  at  the  lowest  current  price. 


7  Lantz,  D.  E.  U.  S.  Dept.  Agric.  Yearbook,  Separate  no.  708,  p.  7,  1916. 


CIRC.  296]  CONTROL  OF  THE  CALIFORNIA  GROUND  SQUIRREL  15 


BOUNTY  SYSTEM   A    FAILURE 

Some  variety  of  bounty  system  intended  to  secure  the  eradication 
of  injurious  rodents  such  as  the  ground  squirrel  has  been  tried  in 
Tulare  and  in  other  counties  in  California,  but  in  most  cases  it  has 
proved  far  from  satisfactory.  Such  a  system  usually  results  in  an 
early  depletion  of  the  funds  provided  to  pay  the  bounty.  When  this 
occurs  the  animals  are  soon  permitted  to  regain  their  former  numbers. 
Bounties  have  not  resulted  in  extermination  of  the  animals  aimed 
at  in  any  of  the  several  states  where  tried.  A  bounty  high  enough 
to  secure  extermination  would  be  prohibitive  on  account  of  the  cost, 
as  no  state  or  county  could  stand  the  financial  strain  for  any  great 
length  of  time.  A  lower  bounty  means  that  the  animals  will  be 
trapped  only  so  long  as  it  is  profitable,  after  which  they  will  be  left 
to  breed  up  again.  L.  B.  Nagler,  Assistant  Secretary  of  State  in 
Wisconsin  says:8  "I  have  had  ten  years'  experience  in  auditing 
bounty  claims,  and  the  results  convince  me  that  the  system  in  vogue 
is  not  only  ineffective  but  wastef ul  and,  in  a  large  measure,  harmful. ' ' 

It  is  the  opinion  of  those  best  informed  on  the  subject,  that  the 
bounty  system  is  not  only  vastly  expensive  and  productive  of  endless 
fraud,  but  that  it  fails  to  accomplish  the  end  desired. 


s  Fins,  Feathers  and  Fur,  June,  1917,  p.  1. 


20m-9,'25 


